The modern internal combustion engine relies on a relatively complex emissions control system to reduce the volume of undesirable tailpipe gases. One of the important targets of the emission control system is the control of NOx formed in the combustion chamber of the engine. The NOx is the result of the combination of oxygen and nitrogen burned at a high temperature.
Modern internal combustion engines typically rely on Exhaust Gas Recirculation (EGR) to reduce NOx emissions. The EGR system recirculates a portion of the exhaust back into the intake air for re-combustion. The EGR system includes a gas pressure sensor and an exhaust gas pressure tube positioned between the exhaust manifold and the exhaust gas pressure sensor. Working in a high heat environment (with temperature reaching about 900° C.), the exhaust gas pressure tube separates the exhaust gas pressure sensor from the heat of the exhaust manifold.
According to known techniques, exhaust gas pressure tubes are manufactured by first flaring the open end of the tube followed by positioning a thrust nut on the tube. The thrust nut of the exhaust gas pressure sensor tube assembly is then threaded into a boss formed on the exhaust manifold. The flare on the exhaust gas pressure tube positioned against the boss seals the tube relative to the exhaust manifold.
The known techniques present increasing challenges to the installation of the exhaust gas pressure tube in the engine compartment which is increasingly crowded due to the ever-increasing number of components necessary for proper engine operation and emission control. It is highly desirable to provide some way of properly orienting the exhaust gas pressure tube during installation and maintaining that orientation after installation. This is because on an angled exhaust gas pressure tube there is no positional rotational alignment within the tube assembly. The tube could be installed at any rotational angle, thus leaving it up to the installer to properly align the pressure tube between the exhaust manifold boss and the pressure sensor. The result is a potentially inconsistent assembly from vehicle to vehicle. This rotational issue might be resolved if enough pressure on the flare is applied by the thrust nut. However, this result is not always easy to achieve and may result in over-torqueing of the thrust nut and possible damage to the exhaust manifold attachment boss.
To this end, and as part of the present invention, a keyed washer for aligning the exhaust gas pressure tube relative to the exhaust manifold may be included in the assembly. However, simply adding the keyed washer to a tube that was already flared proved to be unsatisfactory due to the imperfect angles formed between the outer surface of the flared end and the adjacent surface of the washer. It was found that, for example, the flared area has a tendency to spring back at least partially toward its pre-flared condition. The net result was a seal that is often susceptible to leakage. Failure of a proper seal being formed between the exhaust gas pressure tube and the exhaust manifold, being an emission joint, could lead to warranty and customer dissatisfaction
Accordingly, known methods of making exhaust gas pressure tubes for emission control systems for vehicles have not always produced satisfactory results. It would be useful to have an exhaust gas pressure tube assembly that provides both a tight seal and proper alignment features for use during installation. As in so many areas of vehicle technology, there is always room for improvement related to exhaust gas pressure tube assemblies used in the automotive vehicle.